1. Technical Field
The present invention generally pertains to motor vehicles. More particularly, the present invention pertains to a system of controlling torque transfer in a motor vehicle and a related method. More specifically, but without restriction to the particular embodiment and/or use which is shown and described for purposes of illustration, the present invention relates to a method and apparatus for automatically and independently transferring torque between the four wheels of a motor vehicle.
2. Discussion
In the increasingly competitive automobile industry, many companies have sought to introduce sport utility vehicles in recent years. Most such vehicles are designed primarily for "on-road" traveling. Various systems for sport utility vehicles are currently utilized for limiting differential speed, or transferring torque, between differentially rotating shafts. For example, it is known to use "on-demand" power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. A four-wheel drive vehicle conventionally employs front and rear drive axles, having front and rear differentials, respectively, for driving each wheel of the vehicle. A torque transfer case is generally used to distribute torque to the front and rear drive axles, and may be provided with a differential for dividing torque in a desired ratio.
A selectively engagable clutch has been used to limit differential rotation between the front and rear axles of the vehicle. The clutch is operative to lock the differential upon sensing a predetermined differential rotation between front and rear output shafts of the transfer case. Activation of the clutch may be controlled by an electronic control system and associated speed sensors measuring speeds of the front and rear output shafts of the transfer case. While an electronic control system can again be useful to limit differential speed between the differentially rotating members, in these examples as well as others, a simplified and less costly mechanical system, retaining advantages of this type of control system would be desirable.
It is also known to incorporate a traction control device having a hydraulic coupling. The hydraulic coupling incorporates a gerotor pump and a multi-plate clutch into a common assembly for transferring torque on-demand in response to a speed differential between the driven and non-driven shafts. Known hydraulic coupling mechanisms have utilized hydraulic pumps which pump fluid in response to relative rotation between two rotating members for purposes of rotatably coupling the two differentially rotating members or shafts. These systems generally include a hydraulic pump coupled to the two differentially rotating shafts, which in turn controls a hydraulically actuated piston. The piston in turn acts on a clutch assembly coupling the two rotating shafts. The hydraulic pump provides volumetric flow of fluid that varies in direct proportion to the relative or differential rotational speeds of the rotating shafts. Generally, the hydraulic piston is equipped with an outlet orifice that restricts the outflow of fluid from the piston in order to generate a back pressure of fluid, which drives the piston to engage the clutch mechanism. Such systems therefore provide a capacity for torque transfer between the rotating members that varies in direct proportion to the relative or differential speed between the two shafts.
Many known systems for controlling wheel slip operate to slow rotation of the faster or slipping wheel. Such systems electronically apply anti-braking system (ABS) technology to reduce the rotational speed of a slipping wheel. While effective for controlling slip, this type of traction control system functions to effectively retard vehicle performance. In this regard, under conditions in which one or more wheels are on non-tractive surfaces and substantially all available torque from the engine is needed to propel the vehicle, such traction control systems for limiting differential speed render the vehicle difficult, if not impossible, to maneuver. For example, where such a vehicle is ascending a substantial grade and three of the wheels are simultaneously on non-tractive surfaces (e.g. snow, ice or mud), the ABS technology is employed to dampen the rotational speed of the slipping wheels (e.g. faster spinning wheels). Effectively only twenty-five percent of the torque produced by the engine is delivered to the remaining non-slipping wheel. Even if the torque delivered to the non-slipping wheel is sufficient to propel the vehicle up the steep grade, the vehicle will not respond efficiently.
Most known sport utility vehicles are designed primarily for "on-road" traveling, while giving the impression of having good "off-road" capability as well. Yet, the need still exists for a sport utility vehicle to achieve uncompromising off-road characteristics, while still satisfying on-road driving expectations to thereby reach a new level of vehicle performance.